The present invention relates to a method and apparatus for extruding a multilayer film, and more particularly to a method for preventing the thermal degradation of the barrier layer component of a multilayer film.
In the manufacture of film materials, it is often desirable to utilize a multilayer film rather than a single layer one. By using a multilayer film, one can combine the properties of different types of materials to form a film having the qualities desired for the end use to which the film is placed.
For example, it is common practice to employ a barrier material, such as a Saran vinylidene chloride-vinyl chloride copolymer, as an inner layer in a multilayer film in order to provide gas and moisture vapor barrier characteristics to the film. Such barrier layer containing multilayer films often have polyolefin outer layers. Arrangements of this type are illustrated by Lee et al U.S. Pat. No. 3,477,099, which is assigned to the same assignee as the instant invention, Komoda et al U.S. Pat. No. 4,410,602, and others.
As disclosed in Lee et al, a problem which exists in forming such multilayer films is that Saran barrier materials, which are halogenated resins, are difficult to extrude because of their corrosive nature. For that reason Lee et al discloses a system for encapsulating an extruded corrosive barrier material in a stream of non-corrosive material and subsequently shaping the stream into a film. The non-corrosive material may be a glue layer, such as an ethylene vinyl acetate, which not only serves to prevent the corrosive barrier material from contacting the surface of the extrusion die, but also helps bond the barrier layer to the outer layers in the resulting multilayer film.
While the system of Lee et al is a definite improvement in production of multilayer film containing a Saran barrier layer, problems still exist in that regard. Some of these problems result from the thermally degradable nature of Saran barrier materials. As a result of their thermal degradability, Sarans have a tendency to "carbonize" in the extrusion apparatus.
Carbonization results in the formation of small carbon particles in the molten extrudate. These carbon particles can result in a carbon build-up in the transfer tube leading from the barrier material extruder to the feed block where various components of the multilayer film are joined together. This carbon build-up in the transfer tube effectively reduces the inside diameter of the transfer tube, thereby creating an undesirable pressure drop and also serving as a source of contamination of the polymer.
Additionally, this carbonization can result in a carbon build-up in the adaptor portion of the feed block where the various polymer streams are joined together prior to extrusion by the coextrusion die. This carbon build-up in the feed block changes the shape of the multilayer film, which in turn creates variations in the thickness of the barrier layer. These variations in the thickness of the barrier layer are undesirable because of their deleterious impact on the physical properties of the film. Finally, this carbon build-up typically requires the manufacturer to shut down and clean the extrusion apparatus about every two weeks. This shut down and cleaning of the extrusion apparatus results in high maintenance costs and lost production time.
It would be desirable, therefore, to overcome the problems discussed above by providing a process and apparatus for extruding a multilayer film wherein the likelihood of carbonization is reduced.